Analog signal processing is an important part of many modern communications systems, such as satellite systems, for example. A received signal from an antenna may include digital or analog information, and it may ultimately be processed digitally, but unless the signal can be digitized directly (a challenging prospect as the frequency of the signal increases), there may be some amount of analog signal processing required. This may include amplification, filtering, transmission over some distance, distribution to multiple receivers/transmitters, and frequency conversion for up- or down-conversion. RF and microwave components are very mature, and a baseline level of performance has been demonstrated for these processing functions. Demand for capacity and the broader use and congestion of the electromagnetic spectrum are among the forces increasing the complexity, cost, and performance requirements of analog systems. As higher levels of performance and higher carrier frequencies become desired, especially in the millimeter wave portion of the spectrum, new approaches may be desirable to meet the challenges. Photonics offers certain advantages in this regard: bandwidth; size, weight and power (SWaP); linearity; frequency agility; and providing a reconfigurable infrastructure for analog signal processing.
Photonic systems may cover a wide frequency range and instantaneous bandwidth (IBW), with frequency ranges extending to millimeter waves and an IBW as large as 4 GHz or more. Optical fiber provides an exceptionally low loss transmission medium, with roughly 0.2 dB/km loss regardless of the analog frequency it is carrying. Wavelength division multiplexing may further extend bandwidth by allowing multiple signals to share the same path.
The ability to rapidly tune a system over wide frequency ranges opens up the useable spectrum, enabling a frequency agile system. A photonic system's frequency range is usually set by either the electro-optic modulator or the photodetector. For each of these components, commercial off-the shelf (COTS) devices exist extending well into the millimeter wave region of the spectrum. Tuning the wavelength of a laser or optical bandpass filter can provide quick access to any portion of the spectrum within the range of these components.
The wide bandwidth and large frequency range of a photonic system may provide a flexible, high frequency backbone that can adapt to changing missions. Such a reconfigurable system may enable flexible architectures, reduce the cost of ownership, and adjust to changing environments. Further background details on photonic frequency conversion systems may be found in Middleton et al., “An Adaptive, Agile, Reconfigurable Photonic System for Managing Analog Signals”, Harris Corporation White Paper, Sep. 10, 2014, which is hereby incorporated herein in its entirety by reference.
Despite the advantages of such photonic systems, next generation digital receivers will likely operate over increasingly wider frequency ranges, and need a relatively high intercept probability. As a result, further improvement in signal identification systems for use with digital receivers may be desirable which can accommodate such broadband operation, yet with desired speed, accuracy, cost, and SWaP.